The present invention relates to a method of producing a silica glass crucible, and especially to a method of producing a silica glass crucible which can improve the yield of semiconductor single crystal when used for pulling of semiconductor single crystal.
As a method of producing semiconductor single crystal such as silicon single crystal, xe2x80x9cthe pulling methodxe2x80x9d (Czochralski method or CZ method), in which seed crystals as cores are immersed in a liquid surface of molten semiconductor materials such that single crystal is grown from the seed crystal, is known. A silica glass crucible is employed for melting semiconductor materials. In recent years, in order to reduce the production cost in the pulling process of single crystal, Multi-pulling method or large-diameter silicon single crystal pulling method has been attempted. However, in the case of such new methods, a silica glass crucible of a large inside or inner diameter are often required.
When the inner diameter of the silica glass crucible is increased and a large amount of semiconductor materials are melted in the silica glass crucible, the melting time becomes longer and thus a longer time is required for entire pulling process. In order to shorten the time requited for the pulling process, increasing the amount of heat (the amount of inputted heat) which is given to the silica glass crucible by a heater is one possible solution. Further, it is preferable that the amount of inputted heat is relatively large for maintaining the temperature of a large amount of the liquid of the semiconductor materials at a predetermined temperature.
However, when the amount of the inputted heat is large, the following undesirable outcomes may occur. It is known that a considerable amount of gaseous components is mixed into the silica glass crucible from the air during the production process and remains as bubbles therein. These bubbles tend to expand when the silica glass crucible is used at a high temperature, and the bubbles present in the transparent layer as the inner surface layer of the crucible, in particular, tend to explode as a result of increase in volume thereof. Pieces of silica glass resulted from such explosions are mixed into the liquid of silicon, transferred in the melt liquid as cristobalite by convection, and may be deposited to the lower end of single crystal of silicon which are in the midst of the pulling process. The single crystal may then collapse from the portion at which the silica glass piece is deposited, thereby deteriorating the yield of the semiconductor single crystal. This undesirable phenomenon becomes more significant when the amount of inputted heat is increased or the heat load is increased as the time required for the pulling process is prolonged, due to use of the silica glass crucible of a large inside diameter. The larger the number or the size of the bubbles present in the transparent layer is, or the larger the volume increasing rate of the bubbles during the single crystal pulling process is, the more easily the bubbles explode.
As a method of producing silica glass which has a highly transparent glass layer having a relatively small amount of bubbles, there exists a known method in which silica glass is produced by melting silica sand powder in a high-temperature atmosphere. Known examples of such a method include what are called the oxygen-hydrogen Verneuil""s method, the arc Verneuil""s method, the plasma-Verneuil""s method and the like, which are different from each other in types of the heat source which forms the high-temperature atmosphere. Attempts have been made so as to apply these melting methods to the production of the silica glass crucible and make substantially eliminate bubbles in the silica glass crucible. For example, in publication of examined patent application No. Hei 4-22861, a method of producing a silica glass crucible is proposed in which a transparent layer is formed in the inner surface portion of the crucible (by using the arc Verneuil""s method, the inner surface portion is made to come into direct contact with the liquid of molten silicon during the pulling process).
In publication of unexamined application No. Hei 8-268727, a method of producing a quartz crucible is disclosed which method includes the steps of: centrifuging silica sand put in a melting pot such that the silica sand has the bowl-like shape; heating the silica sand of bowl-like shape; introducing a rapidly-diffusing gas into the silica sand of bowl-like shape from the outer surface thereof so as to purge remaining gases contained in voids which are formed between every particles of silica sand. In addition, in this method of producing a quartz crucible, vacuum is applied to the bottom of the melting pot of the silica sand such that a flow of the rapidly-diffusing gas is generated, in order to purge the remaining gases from the voids of the silica sand.
In the case of the method of producing a silica glass crucible disclosed in the former of the aforementioned two references, a problem, that the method may not be able to adapt to the current pulling process to a sufficient degree because the heat load is increased or the time required for the pulling process is prolonged due to use of a larger inner diameter of silicon single crystal or introduction of the xe2x80x9cmulti-pullingxe2x80x9d process, may arise. Accordingly, there has been a demand for a silica glass crucible in which bubbles present in the inner surface layer are less likely to explode even when the heat load is relatively large or the time required for the pulling process is relatively long.
According to the method of producing a quartz crucible disclosed in the latter of the aforementioned references, the growth of bubbles in the silica glass crucible during the high-temperature heating process in, for example, manufacturing semiconductor single crystals can be prevented because the gases remaining in the voids of the silica sand are replaced with the rapidly-diffusing gas. However, as the remaining gases (nitrogen gas or oxygen gas) are still not sufficiently replaced with the rapidly-diffusing gas in this production method, bubbles present in the opaque layer tend to increase volume thereof when the crucible is used (i.e., during the pulling process), thereby deteriorating the heat conductivity of the quartz crucible and thus raising the temperature of the quartz crucible. As a result, the bubbles in the transparent layer are also likely to explode.
In general, nitrogen gas and the like trapped in the voids of the silica sand has a larger density than hydrogen gas which is the rapidly-diffusing gas. Accordingly, due to the difference between the density of nitrogen gas and that of hydrogen gas, hydrogen gas simply passes through the voids and thus it takes a long time to complete replacing of nitrogen gas with hydrogen gas. As a result, when the rapidly-diffusing gas is blown into the silica sand for only a short period, the replacement cannot be performed sufficiently and a considerable amount of nitrogen gas and the like are likely to remain in the voids. These remaining gases become the bubbles present in the inner surface layer of the silica glass crucible. In short, according to this method, the number of the bubbles in the crucible remains substantially the same as in the conventional method and cannot be reduced.
In addition, the increase in the amount of inputted heat may also influence the pulling process of semiconductor single crystal. During the pulling process, the silica glass crucible is supported at the outer periphery thereof by a holding member made of graphite, and the holding member is heated by a heater. That is, the semiconductor material in the silica glass crucible is heated by the heater by way of the holding member.
The more transparent the silica glass crucible is, the more effectively the heat from the heater is transferred to the semiconductor materials. However, as a plurality of heater elements are provided with a space between each other, if the silica glass crucible is completely transparent, heat from the heater elements tends to be directly transferred to the semiconductor materials in the silica glass crucible and the semiconductor materials located between the heater elements may not be able to receive heat at a sufficient level. Therefore, in order to make the heat be evenly transferred, an opaque layer containing an appropriate number of bubbles is formed in the vicinity of the outer periphery of the silica glass crucible, such that heat rays emitted from the heater are dissipated in the multiple directions and the distribution thereof is made even when the heat rays pass through the silica glass crucible.
When the number of the bubbles present in the opaque layer or the size of such bubbles is too large, it becomes difficult for the heat rays emitted from the heater to reach the transparent layer side and the liquid of molten semiconductor in the crucible may not be able to receive heat efficiently. When the bubbles increase volume thereof by heating, in particular, heat tends to remain inside the silica glass crucible due to the decreased heat conductivity and the temperature of the silica glass crucible itself may rise up to an extremely high level, in spite that the liquid of molten semiconductor cannot receive sufficient heat.
When the temperature of the silica glass crucible has risen to an extremely high level, devitrification occurs in the vicinity of 1550xc2x0 C. Further, in case the heat from the heater is not sufficiently transferred to the liquid of molten semiconductor material in the silica glass crucible, the temperature of the liquid of molten semiconductor material may be locally dropped, there by causing icing (local solidification).
Therefore, a silica glass crucible, in which the inputted heat effectively acts on the semiconductor materials provided therein and thus the time required for the pulling process can be shortened, any unusual increase in temperature or icing due to the reflection of the heat from the heater toward the heater side is prevented, and the heat from the heater can be evenly transferred to the semiconductor materials and the melt liquid thereof contained in the silica glass crucible, has been demanded.
Further, according to the method of the publication of unexamined application No. Hei 8-268727, as the opening for applying vacuum is provided only at the bottom portion of the mold, it is difficult to make the vacuum degree of inside of the mold higher in a short period of time.
As described above, it is desirable that the number, the size of bubbles present in the inner surface layer of a silica glass crucible, and the volume increasing (expansion) rate of the bubbles (ratio of bubble diameter after/before the pulling process) is reduced, in order to improve the grow th efficiency of single crystals. Further, it is desirable that the bubbles present in the silica glass crucible are less likely to increase volume thereof, so that a stable heat conductivity can be obtained for the outer layer as well.
FIG. 2 shows the results of a research in which the relationship between the average diameter of bubbles and yield of the single crystal in the inner surface layer (1 mm) at the corner portions (i.e., the boundary portions of the bottom portions and the side wall portions) of a silica glass crucible was studied. The corner portion is studied because the corner portion, in particular, experiences a relatively large load during the single crystal pulling process and thus the bubbles present in the corner portion are closely related to the yield of the single crystal.
The silica glass crucible sample used for the research has an inner diameter of 22 inches. The yield was obtained in a condition in which silicon polycrystal of 100 kg was heat melted with maintaining the temperature of the liquid surface at about 1430xc2x0 C., seedbars of silicon single crystals were immersed in the melting surface, and silicon single crystal of 8 inches were pulled up. Test pieces were collected from the aforementioned silica glass crucible and the diameter of the bubbles at the corner portions and the yield were investigated. In the results, as shown in FIG. 2, the yield deteriorated when the average diameter of the bubbles was 200 mm or larger.
An object of the present invention is to provide a method of producing a silica glass crucible, in which: the number, the size, and the expansion ratio of the bubbles present in a transparent layer are made small such that explosion of the bubbles is prevented and the yield of the semiconductor single crystal is increased; and the number, the size, and the expansion ratio of the bubbles present in an opaque layer are set appropriate such that the increase in temperature of the silica glass crucible is suppressed and the heat efficiency during the pulling process of semiconductor single crystal is enhanced.
In the first aspect of the present invention, a method of producing a silica glass crucible, in which silica powder is melted by arc discharge between graphite electrodes and a silica glass crucible is formed in a rotating mold, comprising the steps of supplying at least one type of gas selected from the group consisting of hydrogen, oxygen, water vapor, helium, neon gases to the mold; and passing the silica powder through atmosphere of the at least one type of gas supplied at the previous gas supplying step and then supplying the silica powder to inner surface of the mold. Particularly, in the second aspect of the present invention, hydrogen and oxygen gas are supplied to the mold at previous gas supplying step. In the third aspect of the present invention, the silica powder is dispersed inside the mold, such that the silica powder is softened in atmosphere of the arc discharge prior to the silica powder reaching the inner surface of the mold. In the forth aspect of the present invention, the gas and the silica powder are supplied through a double-wall cylinder, particularly, the silica powder is supplied through an inner cylinder and the gas is supplied through an outer cylinder. In the fifth aspect of the present invention, the silica powder is supplied with oxygen gas through as inner cylinder, and hydrogen gas supplied through an outer cylinder. In the sixth aspect of the present invention, a distal end of the inner cylinder is positioned so as to be retracted with respect to a distal end of the outer cylinder. Furthermore, in the seventh aspect of the present invention, of the silica powder and the gas, at least the silica powder is intermittently supplied, while the arc discharge is continued.
According to these aspects of the present invention, the impurities such as alkaline-earth metal and heavy metals contained in the supplied silica powder are replaced with gases such as hydrogen or combusted in a high-temperature atmosphere, and thus the purity of the silica powder is enhanced. In particular, as the atmosphere in the mold reaches a high-temperature atmosphere, in which the silica powder can melt in a short period of time, due to the supplied oxygen gas and hydrogen gas, the degassing process can be easily conducted, thereby significantly suppressing entry of the bubbles into the product crucible. In addition, as the fine powder of graphite which constitutes the electrode is easily oxidized at a high temperature and released to the ambient air, entry of graphite powder into the product crucible is suppressed. Further, the aforementioned supplied gases are diffused into the product crucible and serves so as to reduce the inner pressure of the bubbles.
In particular, according to the third aspect of the present invention, the silica powder is accumulated, in the softened state, on the melting surface of the silica glass which has already been formed. Therefore, the silica powder comes into easy contact with the melting surface and thus the impurities deposited on the melting silica surface are easily removed.
According to the fourth and the fifth aspects of the present invention, the silica powder is supplied by blowing it with gases. Therefore, the direction of supplying the substances can be controlled by adjusting the direction of the double-wall cylinder. According to the sixth aspect of the present invention, the silica powder is surrounded with gases at the distal end portion of the double-wall cylinder and thus can have excellent contact with the gases. According to the seventh aspect of the present invention, the temperature of the melting layer of the quartz is prevented from dropping, the melting layer continues to react with the atmospheric gases and thus removal of the impurities which could cause generation of the bubbles is further accelerated.
The present invention provide following aspects in making an opaque layer. As an eighth aspect, a method of producing a silica glass crucible, in which silica powder is melted by arc discharge and a silica glass crucible is formed in a rotating mold, comprising the steps of forming an accumulated layer of the silica powder on inner surface of the mold; supplying helium and/or hydrogen (helium represents them hereinafter) gas to the accumulated layer at predetermined positions located in sidewalls and a bottom portion of the mold; starting arc discharge, after supplying helium gas to the accumulated layer for a predetermined time; stopping supply of helium gas and degassing the accumulated layer, when a thin film-like melting layer has been formed on the surface of the accumulated layer; and starting again supply of helium gas when the accumulated layer has reached a predetermined vacuum state.
According to the eighth aspect of the present invention, helium gas is supplied into the voids of the accumulated layer in the vacuum state and the atmosphere in the voids is replaced with helium.
In the ninth aspect of the present invention, putting a cover on the upper opening portion of the mold when the accumulated layer has been formed and degassing the inside of the mold; supplying helium gas to the inside of the mold when the inside of the mold has reached a predetermined vacuum state; and removing the cover and starting arc discharge, after a pressure inside the mold has risen to a predetermined value, wherein, after removing the cover, supply of helium gas is continued for a predetermined time.
According to the ninth aspect of the present invention, as the mold is sealed by a cover, a still higher degree of vacuum can be obtained. Therefore, helium gas which is supplied thereafter is fully spread into the voids of the accumulated layer and thus the replacement by helium gas is sufficiently effected.
In the tenth aspect of the present invention, supplying helium gas to the accumulated layer through predetermined positions located in a sidewall and a bottom portion of the mold; starting arc discharge, after supplying helium gas to the accumulated layer for a predetermined time; continuing supply of helium gas and degassing the accumulated layer through upper portions of the sidewall of the mold, when a thin film-like melting layer has been formed on the surface of the accumulated layer.
Due to this aspect, there arises a flow of helium gas which passes through the accumulated layer from the lower side to the upper side, thereby effecting the replacement action allover the accumulated layer.
In the eleventh aspect of the present invention, the positions through which helium gas is supplied when the thin film-like melting layer has been formed is switched to the upper portions of the sidewall of the mold, and the accumulated layer is degassed through the positions at which helium gas was supplied prior to the arc discharge.
Due to this aspect, there arises a flow of helium gas which passes through the accumulated layer from the upper side to the lower side, thereby effecting the replacement action allover the accumulated layer.
In the twelfth aspect of the present invention, putting a cover on the upper opening portion of the mold when the accumulated layer has been formed and degassing the inside of the mold; supplying helium gas to the inside of the mold through predetermined positions located in sidewalls and a bottom portion of the mold, when the inside of the mold has reached a predetermined vacuum state; and removing the cover and starting arc discharge, after a pressure inside the mold has risen to a predetermined value, wherein, supplying of helium gas is continued and the accumulated layer is degassed at upper portions of the sidewall of the mold, when a thin film-like melting layer has been formed on the surface of the accumulated layer.
In the thirteenth aspect of the present invention, the positions through which helium gas is supplied when the thin film-like melting layer has been formed is switched to the upper portions of the sidewall of the mold, and the accumulated layer is degassed through the positions at which helium gas was supplied prior to the arc discharge.
According to the eighth through eleventh aspects of the present invention, as the voide in the accumulated layer is replaced by helium gas, bubbles mixed in the surface layer of the silica crucible are prevented from expansion at higher temperature atmosphere (1450xcx9c1700xc2x0 C.) during the semi-conductor single crystal pulling process. Therefore, heat conductivity of the silica glass crucible is not go down, then the silica glass crucible is prevented from unusual rise of temperature.
Furthermore, in the fourteenth aspect of the present invention, further comprising the steps of: supplying at least one type of gas selected from the group consisting of hydrogen, oxygen, water vapor, helium, neon gases to the mold; and passing the silica powder through atmosphere of the at least one type of gas supplied at the gas supplying step and then supplying the silica powder to inner surface of the mold.
In the fifteenth aspect of the present invention silica powder is dispersed inside the mold, such that the silica powder is softened in atmosphere of the arc discharge prior to the silica powder reaching the inner surface of the mold.